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Emergent Materials under Extreme Conditions
by Adam Pikul
Emergent Materials vs. Electronic Interactions
Physics of condensed matter is nowadays one of the fastest
developing discipline of modern science. Its attractiveness
is based mainly on a number of glamorous discoveries made in the past
decades. High-transition-temperature superconductivity, exotic
superconductivity, heavy-fermion magnets, quantum dots, non-Fermi-liquids
and quantum criticality, colossal magnetoresistance, magnetically driven
shape-memory effect, new thermoelectric materials, low-dimensional
semiconductors, new forms of carbon (fullerenes and nanotubes),
organic conductors, and many others - all of them had significant impact
on the development of theoretical understanding and led eventually
to numerous technological advances.
Among the very successful implementations of these new phenomena one
can mention such well-known inventions and technologies as i.a.:
high-field electromagnets (superconducting wires), improvements in
the sensitivity of the magnetic read heads used for information storage
(colossal magnetoresistance of hybrid magnetic/metallic systems),
high-resolution color liquid-crystals displays (nanoscopic materials),
high-field permanent magnets (neodymium-based compounds), and long-life
electrical-power sources for spacecrafts (space probes) studying outer
parts of the Solar System (thermoelectric compounds heated by radioactive
materials). In turn, as very promising (while being currently only at the
demonstration stage) one can treat such inventions as, for example:
levitating high-speed trains based on the Meisner effect, bone plates
fabricated using shape memory alloys, or - being today ''in vogue''
- high-durability carbon-nanotubes materials for structural engineering.
In all of the phenomena mentioned above electron-electron interactions
play a key role. These interactions within populations of electrons lead to
emergent collective properties that transcendent the properties of
individual electrons.
Therefore theoretical as well as experimental
studies of the organizing principles governing these new forms of behaviour
are one of the cutting edge frontiers for condensed-matter physics. Those
research are traditionally important due to fundamental issues.
Nevertheless, in addition to its intellectual importance, investigations
of the electronic correlations are becoming today also more and more
economically motivated, as a potential source of new promising materials
for applications. It is also worth noting here, that as many as nine Nobel
prizes have been awarded for work in this field since 1970 (1970, 1972, 1977,
1985, 1987, 1996, 1998, 2003, and 2007).
Strongly Correlated Electron Systems
At the intellectual heart of the present-day research on electronic
properties of condensed matter are so-called strongly correlated electron
systems (SCES).
They contain a large number of intermetallic alloys and
compounds, based on lanthanides and actinides, in which conduction-band
electrons (s, p and d) strongly interact (hybridize)
with electrons of unfilled f shells in a periodic arrangement. Up to
date the compounds with cerium, ytterbium and uranium occupy a major position
in this research area. At high temperatures these materials show local moment
behaviour with only weak coupling to the conduction electrons. Upon cooling
to below a characteristic temperature T* of the order of 10-100 K,
the interaction between the f and conduction electrons becomes progressively
stronger. This Kondo interaction can lead to a complete screening of the
magnetic moments and formation of a new quasi-particles, called ''heavy fermions''.
The Kondo interaction results in a logarithmic increase of the electrical
resistivity upon cooling together with anomalous temperature dependencies
e.g. in the Hall effect, thermoelectric power and magnetoresistance.
Additionally, the f moments are coupled by the
Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction.
The overall low-temperature behaviour of these heavy-fermion systems is determined
by the strength of hybridisation between the f-moments and the
s, p and d conduction electrons, measured by
the exchange integral J.
For low values of J, the RKKY
interaction dominates and thus long-range magnetic ordering occurs. With
increasing J, the Kondo interaction leads to the suppression of
long range magnetic ordering (mostly of an antiferromagnetic type, AFM).
In this case, at the lowest temperatures
T<<T* the f-electrons loose their
localised character and the physical properties can be described in terms
of the Fermi-liquid (FL) theory by assuming the presence of heavy
quasi-particles. The effective mass of these heavy fermions can
be up to three orders of magnitude larger than the free-electron mass.
Finally, for large values of J, an intermediate-valence
state takes place. This scenario implies the existence of a critical value
Jcrit
at which the long-range magnetic order is suppressed to zero temperature.
In the vicinity of this quantum critical point (QCP) the physical properties
show anomalous temperature dependencies in a wide range of temperature,
in strong contradiction to Fermi-liquid theory. This so-called
non-Fermi-liquid behaviour (NFL) is nowadays a matter of intensive studies
in a number of scientific centers all over the world.
All those unusual behaviours of SCES can be evidenced only at very low temperatures, where the phonon contribution to the physical quantities studied is neglectible. In order to characterise the physical properties, magnetic, electrical-transport and thermodynamic measurements are important. Among these different techniques, the specific heat at low and ultra-low temperatures is crucial for the correct description of the different ground states in SCES.
Due to their quantum character, the strong electronic correlations are extremely sensitive on application of high magnetic field and/or high hydrostatic pressure. These two external parameters can significantly modify the electronic structure of the materials studied (in particular: the magnitude of the hybridisation) and result in changing their physical properties.
In such a way, for instance, pressure-induced heavy-fermion superconductivity and field-induced non-Fermi-liquid behaviour were discovered. It happens, that the application of an external parameter can result in the appearance of completely new physical phenomena...
On-going projects
- Novel Kondo and heavy-fermion compounds
- Novel non-Fermi-liquid systems
- Novel strongly correlated thermoelectric materials
